Hi Dodgealum
In the sub $500 category I say look at the Grant Fidelity DAC-11 for your 650s. I enjoyed my Sennheiser headphones with Grant Fidelity DAC-09 with a Western Electric 396A tube in place of the stock one.
From other people I've spoke too they liked pairing the Schiit Bifrost DAC and the Schiit Valhalla headphone amp with their Sennheisers. That combo new is under $1K. In the used market you can grab Schiit Audio gear for a lot cheaper.
In the sub $500 category I say look at the Grant Fidelity DAC-11 for your 650s. I enjoyed my Sennheiser headphones with Grant Fidelity DAC-09 with a Western Electric 396A tube in place of the stock one.
From other people I've spoke too they liked pairing the Schiit Bifrost DAC and the Schiit Valhalla headphone amp with their Sennheisers. That combo new is under $1K. In the used market you can grab Schiit Audio gear for a lot cheaper.